Alkyd resins are a well understood and dominant binder in many oxidatively curable paints and other solvent-based coatings. Alkyd emulsion paints, in which the continuous phase is aqueous, are also widely available commercially. Alkyd resins are produced by the reaction of polyols with carboxylic acids or anhydrides. To make them susceptible to what is commonly referred to as a drying process, some alkyd resins are reacted with unsaturated triglycerides or other source of unsaturation. Plant and vegetable oils, such as linseed oil, are frequently used as the source of triglycerides. In these drying processes, unsaturated groups, in particular carbon-carbon double bonds, can react with oxygen from the air, causing the oils to crosslink, forming a three-dimensional network, and harden. This oxidative curing process, although not drying, gives the appearance of drying and is often and herein referred to as such. The length of time required for drying depends on a variety of factors, including the constituents of the alkyd resin formulation and the amount and nature of the liquid continuous phase (e.g., solvent) in which the alkyd resin is formulated.
Film formation results from the autoxidation and polymerization chemistries that occur during the drying of alkyd-based resins. It will proceed in the absence of catalysis. However, it is customary to include in formulations of curable resins small, i.e. catalytic, quantities of optionally organic metal salts, often referred to as metal driers, which catalyze the polymerization of unsaturated material so as to form the three-dimensional network.
Driers used for solvent-based coatings typically include alkyl carboxylates, typically C6-C18 carboxylates, of metals such as cobalt, manganese, lead, zirconium, zinc, vanadium, strontium, calcium and iron. Such metal carboxylates are often referred to as metal soaps. Redox-active metals, such as manganese, iron, cobalt, vanadium and copper enhance radical formation, and thus the oxidative curing process, whilst so-called secondary driers (sometimes referred to as auxiliary driers), such as complexes based on strontium, zirconium and calcium, enhance the action of the redox-active metals. Often these soaps are based on medium-chain alkyl carboxylates such as 2-ethyl-hexanoate. The lipophilic units in such soaps enhance the solubility of the drier in solvent-based paints and other oxidatively curable coating compositions.
As well as metal soaps, a variety of metal driers that are redox metal complexes containing organic ligands can be used as driers, for example manganese complexes comprising 2,2′-bipyridine.
The formation of a skin or lumpy matter is a problem observed in many oil-based (i.e. organic solvent-based) formulations, and in particular in organic solvent-based alkyd resins, as a consequence of oxidation during storage or transportation. Oxidative polymerization reactions can thus lead to the skin formation before application, as well as the intended drying after application. As alluded to above, these polymerization reactions can be triggered by radicals generated by the action of metal-based driers, for example cobalt-, manganese- or iron-containing driers. In other words, the cause of the skin formation is often associated with the presence of metal driers.
Skin formation during manufacture and storage of air-drying paints and other coatings, in particular of alkyd-based resins, is clearly undesirable. Skin formation can lead to material losses and usage problems, such as surface irregularity after application owing to skin particles remaining in the paint.
Addition of compounds that quench the radicals formed during the storage or transportation processes reduce the skin-forming tendencies of such formulations. Many antiskinning agents are therefore antioxidants. However, addition of such antiskinning antioxidants can also slow the drying desired after application, by reducing the activity of the metal driers.
Oximes, and in particular methylethylketoxime (MEKO), are known to reduce skin formation considerably, particularly with cobalt-based driers. It is understood that the oxime binds to the metal ion during storage of the resin, thereby preventing the metal drier from reacting with oxygen and the substrate for radical formation that otherwise leads to polymerization and skin formation. Upon application of the paint or other coating as a thin layer on a surface, the MEKO can evaporate. In this way, skinning can be prevented or ameliorated, but the cobalt soap can function, after application, as a polymerization catalyst (see J H Bieleman in Additives in Plastics and Paints, Chimia, 56:184 (2002)).
Antiskinning agents, or ways to address the problem of skinning, other than those involving the use of oximes such as MEKO, have been described. For example, WO 00/11090 Al (Akzo Nobel N.V.) describes the use of 1,3-diketones, pyrazoles and imidazoles to reduce the skinning properties; WO 2007/024592 A1 (Arkema Inc.) describes the use of isoascorbate as an antiskinning agent and a co-promoter of metal-based driers; and WO 2008/127739 A1 (Rockwood Pigments NA, Inc.) describes the use of hydroxylamine as an antiskinning agent. Whilst such additives reduce the tendency towards skinning, they can lead to decreased performance of the metal drier if their degree of incorporation is too great and they do not evaporate sufficiently during the coating (e.g., paint) application.
Whilst cobalt driers have been employed for many years as paint driers, there is a desire to develop alternatives, not least since cobalt soaps may need to be registered as carcinogenic materials. Iron- and manganese-based paint driers in particular have received considerable attention in recent years in the academic and patent literature as alternatives to cobalt-based driers. For some recent scientific publications addressing this topic in detail see publications by J H Bieleman (in Additives in Plastics and Paints, Chimia, infra)); J H Bieleman (Marcomol. Symp., 187:811 (2002)); and R E van Gorkum and E Bouwman (Coord. Chem. Rev., 249:1709 (2005)).
WO 03/093384 A1 (Ato B.V.) describes the use of reducing biomolecules in combination with transition-metal salts or complexes based on pyrazoles, aliphatic and aromatic amines, 2,2′-bipyridine, 1,10′-phenanthroline and 1,4,7-trimethyl-1,4,7-triazacyclononane (Me3TACN).
WO 03/029371 A1 (Akzo Nobel N.V.) describes the use of complexes comprising Schiff base compounds to enhance the drying of coatings, in which complexes at least one solubilizing group is covalently bound to the organic ligand.
EP 1382648 A1 (Universiteit Leiden) describes the use of manganese complexes with acetylacetonate and bidentate nitrogen donor ligands in paint drying.
WO 2008/003652 A1 (Unilever PLC et al.) describes the use of tetradentate, pentadentate or hexadentate nitrogen ligands bound to manganese and iron as siccative for curing alkyd-based resins.
Oyman et al. describe the oxidative drying of alkyd paints by [Mn2(μ-O)3(Me3tacn)2](PF6)2(Z O Oyman et al., Surface Coating International Part B—Coatings Transaction, 88:269 (2005)). WO 2011/098583 A1, WO 2011/098584 A1 and WO 2011/098587 A1 (each DSM IP Assets B.V.) describe the use of a variety of dinuclear manganese complexes with Me3TACN as ligand for paint drying.
WO 2012/092034 A2 (Dura Chemicals, Inc.) describes the use of a transition metal and a porphyrin based ligand as a siccative for resin compositions.
The use of mixtures of metal salts and ligands to enhance drying of paint formulations is known. For example, W H Canty, G K Wheeler and R R Myers (Ind. Eng. Chem., 52:67 (1960)) describe the drying capability of a mixture of 1,10-phenanthroline (phen) and Mn soap, which is similar to that of prepared Mn-phen complexes. Mixtures of 2,2′-bipyridine (bpy) and manganese soaps show a better drying performance than manganese soaps without bpy (see P K Weissenborn and A Motiejauskaite, Prog. Org. Coat., 40:253 (2000)). Also, R van Gorkum et al. (Inorg. Chem., 43:2456 (2004)), describe that the addition of bpy to Mn(acetylacetonate)3 gives an acceleration in the drying performance, and attribute this to the formation of manganese-bipyridine complexes. The use of manganese complexes with acetylacetonate and bidentate nitrogen donor ligands in paint drying has also been described in EP 1382648 A1 (Universiteit Leiden).
In WO 2012/093250 A1 (OMG Additives Limited) it is described that, by contacting an aqueous solution of transition metal ions and polydentate ligands with alkyd-based formulations, the resultant formulation shows reduced skinning tendency as compared with the introduction of metal ions and polydentate ligands in nonaqueous media.
It may be inferred from the recent literature, including patent literature, published in the field of oxidatively curable coating formulations, for example from WO 2008/003652 A1, WO 2011/098583 A1, WO 2011/098584 A1, WO 2011/098587 A1 and WO 2012/092034 A2, that advantageous curing rates of oxidatively curable resins, for example alkyd-based resins, results from the use of metal driers comprising ligands that give rise to relatively stable transition metal-ligand complexes. In general, when using polydentate ligands, i.e. ligands that bind a metal ion through more than one donor site, improved stability of the resultant metal complexes in different redox states can be observed as compared to the corresponding complexes were monodentate ligands are used.
Nevertheless, there remains a need in the art of oxidatively curable formulations for the provision of further curable formulations, which need not comprise cobalt-based driers, but which nevertheless exhibit acceptable rates of curing. Also, there remains a need in the field of oxidatively curable alkyd-based formulations to be able to provide a formulation which, on the one hand, ameliorates the problem of skinning upon storage of such formulations that comprise metal-based driers, and on the other hand requires less modification by the manufacturers of oxidatively curable coating compositions suitable for application than existing oxidatively curable alkyd-based formulations that are essentially absent metal-based driers. The present invention is intended to address these needs.